More features added

[jayserdny]

Changes made:

• Middleware to check if a user is a moderator
• Middleware to check if a user is a supervisor
• Fetch all posts from Steem API using the permlinks from the database
• Fetch posts by author from Steem API using the permlinks from the database and author as the query.
• Modularization of the posts endpoint to re-use the same function in different functions.
• Documentation for the endpoints
• Moderation tool: Moderate post.